For a hydrofoil such as the SunriseVI to operate safely, it is important to ensure that the forecast weather conditions are within the operating limits of the vessel. In the event that the vessel encounters severe unpredictable conditions when under way, prudence dictates that the vessel seek safe anchorage or shelter close by. The SunriseVI was designed and built to Russian Class O requirements for operation on rivers, reservoirs and small fresh water lakes. However, the wind and wave conditions which can be encountered during the summer months on Lake Ontario are frequently much more severe than those specified for Russian Class O navigational areas, and the mid-voyage distance to shore or shelter is also much greater. Consequently, there is a much higher risk of encountering conditions that exceed the hydrofoil's design capabilities at a greater distance from a port of safe refuge. Contrary to the weather forecast on the morning of the occurrence, conditions on Lake Ontario steadily worsened until the SunriseVI was caught in waves reaching 2m to 3m in height. Such conditions are well beyond the design limits of 1.3m maximum wave height at foil-borne speeds, and 2.0m maximum wave height in displacement mode. When the vessel's speed was reduced to 15kn, the speed was still higher than that of displacement mode, i.e., less than 11kn. Consequently, the vessel was in the transition zone whereby the vessel's speed was too fast for optimum hydrostatic stability, yet too slow to attain sufficient foil lift for hydrodynamic stability. When the foils became fully submerged, they had a dampening effect on hull motions and the bow was unable to lift as waves higher than 2m were encountered. In following seas, the vessel would have tended to pitch, especially if the forward foil broke clear through a long wave. Also, when running before a following sea, the vessel would tend to dive into the trough of a wave. This would cause the wave to break over the bow onto the front windows of the superstructure. The exact quantity of flood water that entered the passenger compartment is not known. However, based on a reported average of 5cm to 7.5cm of water on the passenger deck (approximately 2m3 to 3m3 of water), it would have caused a reduction in the vessel's stability due to her weight and free surface effect. Immediately following the shattering of the window, the master attempted to regain foil-borne mode. The combination of fluctuating stability encountered in the transition from displacement to foil-borne mode and the free surface effect of the accumulated water on the passenger deck further reduced stability. Reducing the speed to 9kn (full displacement mode) improved stability and control of the vessel; however, this action was taken too late as the window had been broken and the passenger deck partially flooded. The use of a portable bilge pump was not an expedient method to remove the accumulated water from the main deck. Bilge pumps normally operate in wells, and as a result, the pump would not remove all the water from the passenger compartment. The open window that was used as a path overboard for the pump discharge hose posed a risk of further flooding in the prevailing weather conditions. Opening the double-bottom hatch allowed the double-bottom and passenger spaces to become contiguous which, in combination with one open and one broken window, further compromised the vessel's watertight integrity and subdivision.Analysis For a hydrofoil such as the SunriseVI to operate safely, it is important to ensure that the forecast weather conditions are within the operating limits of the vessel. In the event that the vessel encounters severe unpredictable conditions when under way, prudence dictates that the vessel seek safe anchorage or shelter close by. The SunriseVI was designed and built to Russian Class O requirements for operation on rivers, reservoirs and small fresh water lakes. However, the wind and wave conditions which can be encountered during the summer months on Lake Ontario are frequently much more severe than those specified for Russian Class O navigational areas, and the mid-voyage distance to shore or shelter is also much greater. Consequently, there is a much higher risk of encountering conditions that exceed the hydrofoil's design capabilities at a greater distance from a port of safe refuge. Contrary to the weather forecast on the morning of the occurrence, conditions on Lake Ontario steadily worsened until the SunriseVI was caught in waves reaching 2m to 3m in height. Such conditions are well beyond the design limits of 1.3m maximum wave height at foil-borne speeds, and 2.0m maximum wave height in displacement mode. When the vessel's speed was reduced to 15kn, the speed was still higher than that of displacement mode, i.e., less than 11kn. Consequently, the vessel was in the transition zone whereby the vessel's speed was too fast for optimum hydrostatic stability, yet too slow to attain sufficient foil lift for hydrodynamic stability. When the foils became fully submerged, they had a dampening effect on hull motions and the bow was unable to lift as waves higher than 2m were encountered. In following seas, the vessel would have tended to pitch, especially if the forward foil broke clear through a long wave. Also, when running before a following sea, the vessel would tend to dive into the trough of a wave. This would cause the wave to break over the bow onto the front windows of the superstructure. The exact quantity of flood water that entered the passenger compartment is not known. However, based on a reported average of 5cm to 7.5cm of water on the passenger deck (approximately 2m3 to 3m3 of water), it would have caused a reduction in the vessel's stability due to her weight and free surface effect. Immediately following the shattering of the window, the master attempted to regain foil-borne mode. The combination of fluctuating stability encountered in the transition from displacement to foil-borne mode and the free surface effect of the accumulated water on the passenger deck further reduced stability. Reducing the speed to 9kn (full displacement mode) improved stability and control of the vessel; however, this action was taken too late as the window had been broken and the passenger deck partially flooded. The use of a portable bilge pump was not an expedient method to remove the accumulated water from the main deck. Bilge pumps normally operate in wells, and as a result, the pump would not remove all the water from the passenger compartment. The open window that was used as a path overboard for the pump discharge hose posed a risk of further flooding in the prevailing weather conditions. Opening the double-bottom hatch allowed the double-bottom and passenger spaces to become contiguous which, in combination with one open and one broken window, further compromised the vessel's watertight integrity and subdivision. The vessel encountered wave heights of 2m to 3m which exceeded both the EC forecast of 1m waves, and the vessel's operating wave height limitation of 1.3m. As the hydrofoil was operating at a speed higher than that recommended for displacement mode, the bow and forward windows were engulfed by waves. The port forward window failed when it was overstressed by the force of the waves breaking against it, causing water to enter the passenger compartment. The broken window material showed signs of deterioration from exposure to sunlight and was constructed of a material which broke into several large sharp fragments when fractured. The accumulation of water shipped and retained on the passenger deck reduced the stability of the vessel. Attempting to regain foil-borne mode with an accumulation of water on the passenger deck further reduced the stability of the vessel. The procedures used to operate the portable pump carried on board (instead of a fixed bilge pumping system) compromised the watertight integrity and subdivision of the vessel. Following the damage, the vessel's speed was reduced to the slowest (approximately 9kn) preventing further water from entering the passenger compartment during the remainder of the voyage. The vessel was not built to a standard of subdivision which complied with the damage stability requirements of the DSC Code; however, she was modified by the addition of polystyrene-filled plastic bags in several compartments. The Voskhod-2 class of hydrofoil is not designed for operation in the weather conditions that may be encountered on the open waters of Lake Ontario.Findings The vessel encountered wave heights of 2m to 3m which exceeded both the EC forecast of 1m waves, and the vessel's operating wave height limitation of 1.3m. As the hydrofoil was operating at a speed higher than that recommended for displacement mode, the bow and forward windows were engulfed by waves. The port forward window failed when it was overstressed by the force of the waves breaking against it, causing water to enter the passenger compartment. The broken window material showed signs of deterioration from exposure to sunlight and was constructed of a material which broke into several large sharp fragments when fractured. The accumulation of water shipped and retained on the passenger deck reduced the stability of the vessel. Attempting to regain foil-borne mode with an accumulation of water on the passenger deck further reduced the stability of the vessel. The procedures used to operate the portable pump carried on board (instead of a fixed bilge pumping system) compromised the watertight integrity and subdivision of the vessel. Following the damage, the vessel's speed was reduced to the slowest (approximately 9kn) preventing further water from entering the passenger compartment during the remainder of the voyage. The vessel was not built to a standard of subdivision which complied with the damage stability requirements of the DSC Code; however, she was modified by the addition of polystyrene-filled plastic bags in several compartments. The Voskhod-2 class of hydrofoil is not designed for operation in the weather conditions that may be encountered on the open waters of Lake Ontario. The SunriseVI took on water when the vessel's bow was engulfed by waves two to three metres in height causing a forward window to be stove in. Contributing to the occurrence was the vessel's high speed and her unsuitability for service on the exposed waters of Lake Ontario where wave heights frequently exceed the vessel's safe operating limits.Causes and Contributing Factors The SunriseVI took on water when the vessel's bow was engulfed by waves two to three metres in height causing a forward window to be stove in. Contributing to the occurrence was the vessel's high speed and her unsuitability for service on the exposed waters of Lake Ontario where wave heights frequently exceed the vessel's safe operating limits. Safety Action